Method for recovering metallic nuclear fuel materials from spent nuclear fuel and method for reprocessing spent nuclear fuel

ABSTRACT

A spent oxide form nuclear fuel in a spent nuclear fuel assembly which has been taken out from a light water reactor is reacted with fluorine in fluorination treatment process and then separated into gaseous UF 6  and solid converted fluoride. The UF 6  is purified in UF 6  treatment Process. In electrolysis using fused fluoride process, the converted fluoride is dissolved into a fused fluoride salt (a mixture of LiF and BeF 2 ) filled into an electrolysis cell of an apparatus for electrolysis. A first electrode, which is an anode, and a second electrode, which is a cathode, are submerged into the fused fluoride. A mixture of the oxides Li 2 O and BeO are added to the fused fluoride. A metallic plutonium and a metallic uranium contained in the fused fluoride is deposited onto the second electrode by energizing of the first and second electrodes.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent applicationserial no. 2008-298918, filed on Nov. 25, 2008, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a method for recovering metallicnuclear fuel material from spent nuclear fuel and a method forreprocessing spent nuclear fuel, and more particularly, to a method forrecovering metallic nuclear fuel material from spent nuclear fuel and areprocessing method for reprocessing spent nuclear fuel, suitable forobtaining metallic nuclear fuel.

Plutonium (mainly plutonium 239) is generated by neutron absorption ofuranium 238 in a core of a light water reactor in which uranium is usedas the nuclear fuel material. Spent fuel assemblies that are loaded intothe light water reactor are taken out from the light water reactor andthen reprocessed. A fast breeder reactor exists as a nuclear reactorwhich generates electricity by using as nuclear fuel material theplutonium recovered by the reprocessing of the spent fuel assemblies andwhich generates at the same time more plutonium than the loadedplutonium. Among this fast breeder reactor is type of a reactor whichuses metallic nuclear fuel containing uranium and plutonium as nuclearfuel materials.

The spent fuel assemblies taken out from the fast breeder reactor loadedfuel assemblies including the metallic nuclear fuel are reprocessed by afused-salt electrolysis method, thereby providing reuse of recovered Puformed into metallic nuclear fuel (see, for example, Masashi Koyama etal., “Dry Reprocessing Technology”, Denchuken Review, No. 37, pp. 26-37(2000)). Nevertheless, a great majority of the nuclear reactors incurrent operation are light water reactors which use an oxide formnuclear fuel as the nuclear fuel material. Spent fuel assemblies takenout from the light water reactors include oxide form nuclear fuelscontaining about 1% Pu. Accordingly, an example of producing from oxideform nuclear fuels a metallic nuclear fuel which is used in a fuelassembly which is originally loaded into a fast breeder reactor usingthe metallic nuclear fuel is described by Tsuyoshi Usami et al. in“Adoption of Dry Reprocessing Technology for Oxide Form Nuclear Fuel”,Denchuken Review, No. 37, pp. 40-46 (2000). In this manufacturing methodof the metallic nuclear fuel, uranium oxide in the spent fuel assembliesused in light water reactors is reduced by lithium; thereby, metallicuranium produced is recovered.

As a method for reprocessing spent nuclear fuel, using as fluoridesuranium and plutonium contained in spent nuclear fuel materials,separating uranium, and uranium and plutonium mixtures by utilizing thedifference in their volatilities, and recovering them has been proposed(Japanese Patent Laid-open No. 2000-284089). This reprocessing methodvolatizes a large portion of the uranium contained in a spent nuclearfuel material by using a fluorination treatment that uses a firstfluorination agent and then separates the large portion of the uranium.Later, the remaining uranium and plutonium are volatized by using asecond fluorination agent and then recovered, followed by oxidation ofthe recovered uranium and plutonium. In this way, mixtures of uraniumand plutonium are generated.

Japanese Patent Laid-open No. 2004-233066 also discloses a reprocessingmethod for spent nuclear fuel by way of a fluorination treatment. Thisreprocessing method reacts spent nuclear fuel materials with fluorinegas and volatizes them in the form of UF₆ and PuF₆. Gas mixed with theseis supplied to a Pu recovery trap loaded with pelletized UO₂F₂, PuF₆ isadsorbed in the form of PuF₄ onto UO₂ and then separated. UF₆ istransformed into UO₂, and UO₂F₂ and PuF₄ are transformed into a mixedoxide of UO₂ and PuO₂.

Japanese Patent Laid-open No. 2002-257980 discloses a reprocessingmethod for spent nuclear fuel, which uses a PUREX process to obtain amixed oxide nuclear fuel containing uranium and plutonium obtained froma spent nuclear fuel.

Another reprocessing method, which uses a fused salt electrolysismethod, for the spent nuclear fuel is explained in Japanese PatentLaid-open No. 2003-43187. An oxide form nuclear fuel that is a spentnuclear fuel material is charged into fused salt filled in a vessel andchlorine gas is blown into the fused salt. UO₂ and PuO₂ contained in theoxide form nuclear fuel are dissolved as chlorides into the fused salt.An anode and a cathode are soaked into the fused salt. When a current ispassed between the anode and the cathode, UO₂ and PuO₂ are depositedonto the cathode. A reprocessing method for spent nuclear fuel, whichuses a fused salt electrolysis method is also disclosed in JapanesePatent Laid-open No. 2000-284089.

SUMMARY OF THE INVENTION

In order to obtain metallic nuclear fuel for use in fuel assemblies tobe loaded into fast breeder reactors, the inventors studied reprocessingmethod for oxide form nuclear fuels included in spent nuclear fuelassemblies taken out from light water reactors. As a result, in order toobtain metallic nuclear fuel from spent oxide form nuclear fuel, theinventors arrived at the conclusion that the use of a fused-saltelectrolysis method would be good.

However, amount of uranium contained in metallic nuclear fuel present infresh fuel assemblies that are loaded in the core of the fast breederreactor is 3 to 4 times amount of plutonium in the metallic nuclearfuel, contrasting to amount of uranium contained in oxide form nuclearfuel in spent fuel assemblies taken out from light water reactor, whichis about 100 times amount of plutonium contained in the oxide formnuclear fuel. Therefore, prior to process of fused salt electrolysis, alarge portion of the excess uranium must be removed from spent oxideform nuclear fuel.

In order to remove the excess uranium prior to the fused saltelectrolysis process used to obtain the metallic nuclear fuel, theinventors gave thought to subjecting spent oxide form nuclear fuel arepreviously subjected to a fluorination process, as is disclosed inJapanese Patent Laid-open No. 2000-284089, Japanese Patent Laid-open No.2004-233066, and Japanese Patent Laid-open No. 2002-257980. Byconducting previously the fluorination process on spent oxide formnuclear fuel, a large portion of the uranium is converted into afluoride, volatized, and removed. Additionally, as disclosed in JapanesePatent Laid-open No. 2000-284089 and Japanese Patent Laid-open No.2004-233066, each fluoride of the remaining uranium and plutonium areconverted into their respective oxides. The obtained uranium andplutonium oxides are supplied into the fused salt in the vessel andthese oxides are subjected to the fused salt electrolysis. As a result,the metallic nuclear fuel containing metallic uranium and metallicplutonium can be generated. Since excess uranium can be removed fromspent oxide form nuclear fuel, an equipment for conducting the fusedsalt electrolysis on uranium and plutonium can be made compact.Furthermore, it is possible to shorten the time required to obtain themetallic nuclear fuel by the fused salt electrolysis.

The inventors discovered a new issue of the need to further simplify aprocess for recovering from the spent oxide form nuclear fuel themetallic nuclear fuel materials acting as the raw material for metallicnuclear fuel.

An object of the present invention are to provide a method forrecovering metallic nuclear fuel material from spent nuclear fuel and amethod for reprocessing spent nuclear fuel, which can simplify a processfor recovering metallic nuclear fuel from spent oxide form nuclear fuel.

To achieve the above-mentioned object, the present invention ischaracterized by

generating nuclear fuel fluorides by reacting fluorine with spent oxideform nuclear fuel taken out from a nuclear reactor,

removing one part of fluorinated uranium from among the nuclear fuelfluorides,

dissolving the remaining nuclear fuel fluorides and oxides into fusedfluoride, and

energizing a first electrode which is an anode and a second electrodewhich is a cathode, both of which were immersed into the fused fluoride,and depositing onto the second electrode a metallic nuclear fuelmaterial dissolved in the fused fluoride.

Since the nuclear fuel fluorides and oxides are dissolved in the fusedfluoride, and the metal nuclear fuel material dissolved in the fusedfluoride is deposited onto the second electrode by energizing the firstelectrode which is the anode and the second electrode which is thecathode, both of which were immersed into the fused fluoride, a process,which is a pretreatment for electrolysis using fused fluoride, forconverting the nuclear fuel fluorides into oxides becomes unnecessary.Accordingly, the process of the method for recovering the metallicnuclear fuel materials from the spent oxide form nuclear fuels can besimplified.

According to the present invention, the process for recovering themetallic nuclear fuel materials from the spent oxide form nuclear fuelcan be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing a process of a method forrecovering metallic nuclear fuel from the spent nuclear fuel accordingto Embodiment 1 which is one suitable embodiment of the presentinvention.

FIG. 2 is an explanatory drawing showing material balance of nuclearfission products in the embodiment shown in FIG. 1.

FIG. 3 is a characteristic drawing showing a relationship between numberof treatments of a process of electrolysis using fused fluoride andamount of nuclear fission products transferred to a treatment process ofwaste salt from an electrolysis cell.

FIG. 4 is an explanatory drawing showing a process of a method forrecovering spent nuclear fuel according to Embodiment 2, which isanother embodiment of the present invention.

FIG. 5 is an explanatory drawing showing a process of electrolysis usingfused fluoride in a method for recovering spent nuclear fuel accordingto Embodiment 3, which is another embodiment of the present invention.

FIG. 6 is an explanatory drawing showing a purification process in amethod for recovering spent nuclear fuel according to Embodiment 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors investigated of a discovered new process for reprocessingspent oxide form nuclear fuels, that is, a treatment flow of afluorination treatment for spent oxide form nuclear fuel, convertingrespective fluorides of uranium and plutonium obtained thereby intorespective oxides of uranium and plutonium, and fused salt electrolysisof the respective oxides of uranium and plutonium. As a result, theinventors found knowledge that it is possible to further simplify theprocess of reprocessing used oxide form nuclear fuels by fused saltelectrolysis of the respective fluorides of uranium and plutoniumobtained by a fluorination treatment of oxide form nuclear fuels. Inorder to realize this knowledge, (a) use of a fused fluoride acting asfused salt used in the fused salt electrolysis, and (b) the need to addan oxide to the fused fluoride were newly discerned.

As the fused fluorides, for example, a mixture of LiF and BeF₂ is usedand, as the added oxides, a mixture of Li₂O and BeO is used. By doingso, fused salt electrolysis of the respective fluorides of uranium andplutonium obtained by the fluorination treatment of the oxide formnuclear fuel is made possible. Accordingly, conversion treatment of therespective fluorides of uranium and plutonium into oxides, which ispretreatment for fused salt electrolysis, becomes unnecessary, and inthe method for recovering metallic nuclear fuel materials from usedoxide form nuclear fuels, the process of recovering the metallic nuclearfuel materials from spent oxide form nuclear fuels can be simplified.

Reflecting the abovementioned investigation results, embodiments of thepresent invention are explained below.

Embodiment 1

A method for recovering metallic nuclear fuel materials from spentnuclear materials according to embodiment 1 which is one suitableembodiment of the present invention, is explained below by referring toFIG. 1.

Spent fuel assemblies being loaded into a core of a light water reactorare taken out from a reactor pressure vessel of the light water reactorand stored for a specified period in a fuel storage pool. This spentfuel assemblies are then taken out from the fuel storage pool andtransferred to a nuclear fuel reprocessing facility from the nuclearpower generation facility where the light water reactor is located.Nuclear fuel materials, that is oxide form nuclear fuels, contained inthe spent fuel assemblies are reprocessed in the nuclear fuelreprocessing facility.

TABLE 1 Spent oxide form nuclear fuel Converted fluoride ComponentAmount of Amount of Line Atomic Mass Radioactivity substance MassRadioactivity substance number Element weight Form (kg) (Ci) (kmol) Form(kg) (Ci) (kmol) 1 U 238.029 UO₂ 9.54E+02 4.05E+00 4.01E+00 UO₂F₂1.91E+01 8.10E−02 8.02E−02 2 Pu 244 PuO₂ 9.03E+00 1.08E+05 3.70E−02 PuF₄9.03E+00 1.08E+05 3.70E−02 3 Np 237.0482 NpO₂ 7.49E−01 1.81E+01 3.16E−03NpO₂F₂ 7.49E−01 1.81E+01 3.16E−03 4 Am 243 Am₂O₃ 1.40E−01 1.88E+025.76E−04 AmF₃ 1.40E−01 1.88E+02 5.76E−04 5 Cm 247 Cm₂O₃ 4.70E−021.89E+04 1.90E−04 CmF₃ 4.70E−02 1.89E+04 1.90E−04 6 H 1.0079 H₂O7.17E−05 6.90E+02 7.11E−05 — 0.00E+00 0.00E+00 0.00E+00 7 Se 78.96 SeO₂4.87E−02 3.96E−01 6.17E−04 SeF₄ 4.87E−02 3.96E−01 6.17E−04 8 Br 79.904Br₂ 1.38E−02 0.00E+00 1.73E−04 — 0.00E+00 0.00E+00 0.00E+00 9 Kr 83.8 Kr3.60E−01 1.10E+04 4.30E−03 — 0.00E+00 0.00E+00 0.00E+00 10 Rb 85.4678Rb₂O 3.23E−01 1.90E+02 3.78E−03 RbF 3.23E−01 1.90E+02 3.78E−03 11 Sr87.62 SrO 8.68E−01 1.74E+05 9.91E−03 SrF₂ 8.68E−01 1.74E+05 9.91E−03 12Y 88.9059 Y₂O₃ 4.53E−01 2.38E+05 5.10E−03 YF₃ 4.53E−01 2.38E+05 5.10E−0313 Zr 91.22 ZrO₂ 3.42E+00 2.77E+05 3.75E−02 ZrF₄ 3.42E+00 2.77E+053.75E−02 14 Nb 92.9064 Nb₂O₅ 1.16E−02 5.21E+05 1.25E−04 NbF₅ 1.16E−035.21E+04 1.25E−05 15 Mo 95.94 MoO₃ 3.09E+00 0.00E+00 3.22E−02 MoF₆3.09E−01 0.00E+00 3.22E−03 16 Tc 97 TcO₂ 7.52E−01 1.43E+01 7.75E−03 TcF₆7.52E−02 1.43E+00 7.75E−04 17 Ru 101.07 RuO₂ 1.90E+00 4.99E+05 1.88E−02RuF₆ 1.90E−01 4.99E+04 1.88E−03 18 Rh 102.9055 Rh₂O₃ 3.19E−01 4.99E+053.10E−03 RhF₃ 3.19E−01 4.99E+05 3.10E−03 19 Pd 106.4 Pb₂O₃ 8.49E−010.00E+00 7.98E−03 PdF₃ 8.49E−01 0.00E+00 7.98E−03 20 Ag 107.868 Ag₂O4.21E−02 2.75E+03 3.90E−04 AgF 4.21E−02 2.75E+03 3.90E−04 21 Cd 112.4CdO 4.75E−02 5.95E+01 4.23E−04 CdF₂ 4.75E−02 5.95E+01 4.23E−04 22 In114.82 In₂O₃ 1.09E−03 3.57E−01 9.49E−06 InF₃ 1.09E−03 3.57E−01 9.49E−0623 Sn 121.75 SnO 3.28E−02 3.85E+04 2.69E−04 SnF₄ 3.28E−02 3.85E+042.69E−04 24 Sb 121.75 Sb₂O₅ 1.36E−02 7.96E+03 1.12E−04 SbF₅ 1.36E−037.96E+02 1.12E−05 25 Te 127.6 TeO₃ 4.85E−01 1.34E+04 3.80E−03 TeF₆4.85E−02 1.34E+03 3.80E−04 26 I 126.9045 I₂ 2.12E−01 2.22E+00 0.00E+00 —0.00E+00 0.00E+00 0.00E+00 27 Xe 131.3 Xe 4.87E+00 3.12E+00 0.00E+00 —0.00E+00 0.00E+00 0.00E+00 28 Cs 132.9054 Cs₂O 2.40E+00 3.21E+051.81E−02 CsF 2.40E+00 3.21E+05 1.81E−02 29 Ba 137.34 BaO 1.20E+001.00E+05 8.74E−03 BaF₂ 1.20E+00 1.00E+05 8.74E−03 30 La 138.9055 La₂O₃1.14E+00 4.92E+02 8.21E−03 LaF₃ 1.14E+00 4.92E+02 8.21E−03 31 Ce 140.12Ce₂O₃ 2.47E+00 8.27E+05 1.76E−02 CeF₃ 2.47E+00 8.27E+05 1.76E−02 32 Pr140.9077 Pr₂O₃ 1.09E−01 7.71E+05 7.74E−04 PrF₃ 1.09E−01 7.71E+057.74E−04 33 Nd 144.24 Nd₂O₃ 3.51E+00 9.47E+01 2.43E−02 NdF₃ 3.51E+009.47E+01 2.43E−02 34 Pm 145 Pm₂O₃ 1.00E−01 1.00E+05 6.90E−04 PmF₃1.00E−01 1.00E+05 6.90E−04 35 Sm 150.4 Sm₂O₃ 6.96E−01 1.25E+03 4.63E−03SmF₃ 6.96E−01 1.25E+03 4.63E−03 36 Eu 151.96 Eu₂O₃ 1.26E−01 1.35E+048.29E−04 EuF₃ 1.26E−01 1.35E+04 8.29E−04 37 Gd 157.25 Gd₂O₃ 6.29E−022.32E+01 4.00E−04 GdF₃ 6.29E−02 2.32E+01 4.00E−04 38 Tb 158.9254 Tb₂O₃1.25E−03 3.02E+02 7.87E−06 TbF₃ 1.25E−03 3.02E+02 7.87E−06 39 Dy 162.5Dy₂O₃ 6.28E−04 0.00E+00 3.86E−06 DyF₃ 6.28E−04 0.00E+00 3.86E−06 40Total amount 9.94E+02 4.54E+06 4.27E+00 4.79E+01 3.60E+06 2.81E−01 41MA + FP 3.09E+01 4.44E+06 2.25E−01 1.98E+01 3.49E+06 1.64E−01 42 FP2.99E+01 4.42E+08 2.21E−01 1.88E+01 3.47E+06 1.60E−01

After the oxide form nuclear fuel are removed from the nuclear fuel rodsby removing claddings and then pulverized, a fluorination treatment isapplied to the spent oxide form nuclear fuels (fluorination treatmentprocess 1). When 1 ton of the oxide form nuclear fuel is loaded into thecore of the light water nuclear reactor, at the point in time when oneoperation cycle is finished, the amount of the oxide form nuclear fuelhas reduced to 0.994 ton due to nuclear fission occurring duringoperation in the light water reactor, as shown in Table 1. The spentoxide form nuclear fuel included in the spent fuel assemblies taken outfrom the light water reactor have compositions shown in a section of thespent oxide form nuclear fuel shown in Table 1, and contained, as themajor components, uranium; transuranic elements such as plutonium,neptunium, americium, and curium, generated from the uranium; andfission products (hereinafter called “FP”). In a section of the spentoxide form nuclear fuel shown in Table 1, the weight, masses (number ofmoles), and radioactivity of each element contained in about 1 ton ofthe spent oxide form are shown.

In fluorination treatment process 1, the oxide form nuclear fuel arereacted with fluorine and then divided into gaseous uranium hexafluoride(UF₆) and a solid converted fluoride. The UF₆ generated in fluorinetreatment process 1 is transferred to UF₆ treatment process 2 and thenrefined and converted into UO₂. This UO₂ is filled into fuel rods forflesh nuclear fuel assemblies for the light water reactors.

A flame reactor (or fluidized bed) is used in fluorination treatmentprocess 1. After the oxide form nuclear fuel are supplied to the flamereactor, fluorine is supplied to the flame reactor and fluorinationtreatment is conducted on the oxide form nuclear fuel within the flamereactor. A percentage of uranium being volatized in the fluorinationtreatment process 1 can be adjusted between from 80% to 98% bycontrolling the amount of fluorine (F₂) supplied to the flame reactor.

The amount of uranium remaining in the solid converted fluoride can beadjusted by adjusting the percentage of uranium volatilization. Thecompositions of the converted fluorides obtained by applying thefluorination treatment to the spent oxide form nuclear fuel having theconstitutions disclosed in the section of the spent oxide form nuclearfuel of Table 1 and by making volatilization of 98% of the uranium, areshown in a section of the converted fluoride of Table 1. Each compoundcontained in the spent oxide form nuclear fuel prior to the fluorinationtreatment is converted after the fluorination treatment into each of thecompounds shown in a section of the form in the section of the convertedfluoride of Table 1. Due to being removed as UF₆ during fluorinationtreatment, a large portion of the uranium is reduced to 2% of the amountof uranium contained in the spent oxide form nuclear fuel. Furthermore,H, Br, Kr, I and Xe contained in the spent oxide form nuclear fuel,since they are gasses, are removed in the fluorination treatment process1, and are no longer contained in the converted fluorides which weregenerated in the fluorination treatment process 1. Since the fluoridesof Se, Nb, Tc, Mo, Ru, and Sb are volatile, only about 10% of themremain in the converted fluorides.

The fluorination treatment process which uses a fluidized bed occursbatchwise, so at the point in time when just the essential amount ofuranium is volatized, supply of fluorine to the fluidized beddiscontinues, and the remaining substance is removed from the fluidizedbed. By conducting such an operation, the percentage of uraniumvolatized can be adjusted even when a fluidized bed is used.

After the abovementioned fluorination treatment process 1 was finished,a process 3 of electrolysis using fused fluoride is carried out.Converted fluoride generated in the fluorination treatment process 1 issupplied into an electrolysis cell used in the process 3 of electrolysisusing fused fluoride. An apparatus for electrolysis using fusedfluoride, used in the process 3 of electrolysis using fused fluoride inthe electrolysis cell filled with the fused fluoride, and theelectrolysis cell is provided with a first electrode, which is an anode,and a second electrode, which is a cathode. The first and secondelectrodes are submerged into the fused fluoride (see FIG. 5, describedbelow). The converted fluorides are supplied into the fused fluoride. Amixture of LiF and BeF₂ is used, for example, as the fused fluoride. Inplace of the mixture of LiF and BeF₂, a mixture including LiF, NaF, andKF may be used as the fused fluoride.

By using the fused fluoride, the uranium present in the form of UO₂F₂containing the converted fluoride and the neptunium present in the formof NpO₂F₂ containing the converted fluoride is dissolved in the fusedfluoride by the reactions shown below.

UO₂F₂(solid)→UO₂ ²⁺(in fused salt)+2F⁻(in fused salt)  (1)

NPO₂F₂(solid)→NpO₂ ²⁺(in fused salt)+2F⁻(in fused salt)  (2)

Other converted fluorides generated in fluorination treatment process 1are in the form of AnFm, so, for example, they are dissolved into thefused fluoride by the following reaction below.

PuF₄(solid)→Pu⁴⁺(in fused salt)+4F⁻(in fused salt)  (3)

LaF₃(solid)→La³⁺(in fused salt)+3F⁻(in fused salt)  (4)

SrF₂(solid)→Sr²⁺(in fused salt)+2F⁻(in fused salt)  (5)

CsF(solid)→Cs⁺(in fused salt)+F⁻(in fused salt)  (6)

The first electrode and the second electrode are energized and when thefused fluoride in the electrolysis cell is electrolyzed, the reactionsof each of expressions (7) through (10) shown below, occur in the fusedfluoride, and metallic uranium is deposited on the second electrode thatis a cathode. That is, while electricity passes between the electrodes,the fused fluoride generates the reaction in expression (8) due to thehigh degree of electrolysis of O²⁻ in contrast to fused chloride, andthe uranium forms into metallic uranium by passing from U⁴⁺ to U³⁺.

UO₂ ²⁺(in fused salt)+2e ⁻(cathode)→UO₂(near the anode)  (7)

UO₂(near the anode)→U⁴⁺(in fused salt)+2O²⁻(in fused salt)  (8)

U⁴⁺(in fused salt)+e ⁻(cathode)→U³⁺(in fused salt)  (9)

U³⁺(in fused salt)+3e ⁻(cathode)→U(deposition on the anode)  (10)

By summarizing expressions (7) to (10), the reaction arising at thesecond electrode results in expression (11).

UO₂ ²⁺(in fused salt)+6e ⁻→U(deposition on cathode)+2O²⁻(in fusedsalt)  (11)

Similar to UO₂F₂, NpO₂F₂ is deposited on the cathode as Np. Fluoridesexisting in the form of AnFm are, by the following reactions, metalsdeposited on the cathode.

Pu⁴⁺(in fused salt)+4e ⁻(cathode)→Pu(deposited on the cathode)  (12)

La³⁺(in fused salt)+3e ⁻(cathode)→La(deposited on the cathode)  (13)

Sr²⁺(in fused salt)+2e ⁻(cathode)→Sr(deposited on the cathode)  (14)

Cs⁺(in fused salt)+e ⁻(cathode)→Cs(deposited on the cathode)  (15)

The fused fluoride was stated above as being electrolyzed, but as it is,when electrolysis is conducted, A reaction in expression (16) occurs atthe first electrode 1 that is a anode with respect to the reaction inexpression (11) and fluorine gas is generated near the first electrode.Therefore, the first electrode readily corrodes.

6F⁻(in fused salt)→3F₂+6e ⁻(anode)  (16)

In the present embodiment, in order to prevent the generation offluorine near the first electrode, oxide is dissolved in advance intothe fused fluoride in the electrolysis cell. As the oxide to be added tothe fused fluoride, use of an oxide having a cation in common with thefused fluoride is preferable. That is, by using the oxide common to thefused fluoride, for example, as shown below in expression (19), reactingexcess fluorine in the fused fluoride the generated fluoride has thesame composition as the fused fluoride. Therefore, the composition ofthe fused fluoride can be maintained. In the present embodiment, since amixture of LiF and BeF₂ is used as the fused fluoride, a mixture of Li₂Oand BeO is used as the oxide to be added. By using a mixture of Li₂O andBeO, LiF and BeF₂, which are the composition of the fused fluoride usedin the present embodiment, are generated based on the reaction inexpression (19). In the present embodiment, use of a carbon anode as thefirst electrode promotes the reaction of expression (11). Thus, theanode reactions occurring at the first electrode become like those inexpressions (17) and (18).

Li₂O+BeO→2Li⁺(fused salt)+Be²⁺(fused salt)+2O²⁻  (17)

3O²⁻(fused salt)+3/2C(anode material)→3/2CO₂+6e ⁻(anode)  (18)

In the reaction of expression (18), carbon dioxide is generated from thefirst electrode. Nevertheless, depending on the temperature of the fusedfluoride, the generated carbon dioxide may be decomposed and carbonmonoxide may be generated.

In reaction of expression (11) to recover metallic uranium from UO₂F₂ bythe electrolysis using fused fluoride, two O²⁻ are generated, so thereaction of expression (18) arises on the first electrode. Therefore,Li₂O as well as BeO may be added to the mixture of LiF and BeF₂ as thefused fluoride, such that just one mole of O²⁻ are generated per mole ofUO₂F₂. The entire reaction is shown in the following expression.

1/2(Li₂O+BeO)+UO₂F₂+3/2C→Li⁺+1/2Be²⁺+2F⁻+U+3/2CO₂  (19)

Expression (11) shows what materials in the recovered metallic uraniumaccompany. Among the reactions shown in expressions (7) to (10), thereaction which must place the greatest negative value on the secondelectrode is the reaction of expression (10). The electric potential ofthe deposition reaction by the fused fluorides, due to a paucity ofdata, will be explained below, using the electric potential occurringwith fused chlorides.

The electric potentials of the deposition reactions with fused fluorideand fused chloride differ, but the order of the electric potentials todeposit each ion (deposition electric potentials) does not change foreither. That is, although electric potential E1 to make U³⁺ intometallic uranium and electric potential E2 to make La³⁺ into metallic Ladiffer depending on the type of fused salt, even if the fused saltsdiffer, electric potential E2 being a more negative value than theelectric potential E1 does not change.

TABLE 2 Converted fluoride Electrode- Component Amount of position LineAtomic Mass Radioactivity substance potential number Element weight Form(kg) (Ci) (kmol) (V) Remarks 1 Te 127.6 TeF₆ 4.85E−02 1.34E+03 3.80E−04−0.1 2 Ru 101.07 RuF₆ 1.90E−01 4.99E+04 1.88E−03 −0.142 3 Pd 160.4 PdF₃8.49E−01 0.00E+00 7.98E−03 −0.214 4 Rh 102.9055 RhF₃ 3.19E−01 4.99E+053.10E−03 −0.231 5 Ag 107.868 AgF 4.21E−02 2.75E+03 3.90E−04 −0.637 6 Mo95.94 MoF₆ 3.09E−01 0.00E+00 3.22E−03 −0.638 7 Sb 121.75 SbF₅ 1.36E−037.96E+02 1.12E−05 −0.67 8 Sn 121.75 SnF₄ 3.28E−02 3.85E+04 2.69E−04−1.082 9 In 114.82 InF₃ 1.09E−03 3.57E−01 9.49E−06 −1.104 10 Nb 92.9064NbF₅ 1.16E−03 5.21E+04 1.25E−05 −1.19 11 Cd 112.4 CdF₂ 4.75E−02 5.95E+014.23E−04 −1.316 12 Tb 158.9254 TbF₃ 1.25E−03 3.02E+02 7.87E−06 −1.465Approximate with Yb 13 Dy 162.5 DyF₃ 6.28E−04 0.00E+00 3.86E−06 −1.465Approximate with Yb 14 Cm 247 CmF₃ 4.70E−02 1.89E+04 1.90E−04 −1.505 15Am 243 AmF₃ 1.40E−01 1.88E+02 5.76E−04 −1.623 16 Pu 244 PuF₄ 9.03E+001.08E+05 3.70E−02 −1.733 17 Sm 150.4 SmF₃ 6.96E−01 1.25E+03 4.63E−03−1.819 18 Zr 91.22 ZrF₄ 3.42E+00 2.77E+05 3.75E−02 −1.86 19 Np 237.0482NpO₂F₂ 7.49E−01 1.81E+01 3.16E−03 −2.068 20 U 238.029 UO₂F₂ 1.91E+018.10E−02 8.02E−02 −2.253 21 Sr 87.62 SrF₂ 8.68E−01 1.74E+05 9.91E−03−2.58 Approximate with Mg 22 Ba 137.34 BaF₂ 1.20E+00 1.00E+05 8.74E−03−2.58 Approximate with Mg 23 Gd 157.25 GdF₃ 6.29E−02 2.32E+01 4.00E−04−2.823 24 Nd 144.24 NdF₃ 3.51E+00 9.47E+01 2.43E−02 −2.854 25 Y 88.9059YF₃ 4.53E−01 2.38E+05 5.10E−03 −2.866 26 La 138.9055 LaF₃ 1.14E+004.92E+02 8.21E−03 −2.883 27 Pr 140.9077 PrF₃ 1.09E−01 7.71E+05 7.74E−04−2.883 Approximate with La 28 Pm 145 PmF₃ 1.00E−01 1.00E+05 6.90E−04−2.883 Approximate with La 29 Eu 151.96 EuF₃ 1.26E−01 1.35E+04 8.29E−04−2.883 Approximate with La 30 Ce 140.12 CeF₃ 2.47E+00 8.27E+05 1.76E−02−2.94 31 Rb 85.4678 RbF 3.23E−01 1.90E+02 3.78E−03 −3.14 Approximatewith Na 32 Cs 132.9054 CsF 2.40E+00 3.21E+05 1.81E−02 −3.14 Approximatewith Na 33 Se 78.96 SeF₄ 4.87E−02 3.96E−01 6.17E−04 34 Tc 97 TcF₆7.52E−02 1.43E+00 7.75E−04 35 Total amount 4.79E+01 3.60E+06 2.81E−01 36MA + FP 1.98E+01 3.49E+06 1.64E−01 37 FP 1.88E+01 3.47E+06 1.60E−01 38FP mixed in 2.44E+01 1.14E+00 6.12E−02

Table 2 presents a summary, in order from small to large, of theelectric potentials to deposit each compound dissolved in the fusedchloride. Each of the elements in line numbers 1 to 19 has anelectrodeposition potential which is more positive than that of uranium,and deposits more readily than uranium. Therefore, when uranium isdeposited onto the second electrode, each of the elements in linenumbers 1 to 19 is also deposited onto the second electrode. Since theelements in line numbers 21 and below have electrodeposition potentialswhich are more negative than uranium, they deposit less readily thanuranium and when uranium is deposited onto the second electrode, theyare included in the fused salt in the electrolysis cell. Although thereis no data of the electrodeposition potentials for Se and Tc,conservatively, when uranium is deposited onto the second electrode, Seand Tc are also deposited onto the second electrode. Naturally,plutonium is also deposited onto the second electrode.

In the process 3 of electrolysis using fused fluoride of the presentembodiment, the first electrode and the second electrode are energizedsuch that the electric potential difference between the first electrodeand the second electrode, which were provided to the electrolysis cell,becomes equal to the electrodeposition potential of the element(uranium), among the elements required for metallic nuclear fuels to bemanufactured, with the lowest electrodeposition potential within thefused fluoride. By doing so, elements having an electrodepositionpotential equal to or greater than the element with the lowestelectrodeposition potential are deposited onto the second electrode thatis a cathode.

Since 98% of the uranium contained in the spent oxide form nuclear fuelsin the fluorine treatment process 1 has been removed, the proportion(enrichment) of metallic plutonium to the metallic uranium and metallicplutonium deposited on the second electrode becomes 0.32, according toTable 2. That is, according to the present embodiment, after removal ofexcess uranium from the spent oxide form nuclear fuel, the remaininguranium and plutonium can be converted into metallic nuclear fuel havinga plutonium enrichment of at least 0.25. The second electrode, to whichthe recovered metallic nuclear fuel materials (each of the metallicelements, such as metallic uranium and metallic plutonium, of linenumbers 1 to 20 shown in Table 2) adhere, is transported to anotherplant for implementing a purification process 4 set up at a differentlocation, this plant differing from the plant for implementing therecovery method for the metallic nuclear fuel material of the presentembodiment.

TABLE 3 Type of metallic Amount recovered nuclear fuel material(kg/month) Component U 14.66 Pu 6.936 MA 0.719 RE 0.536 AM 0.000 AEM0.000 NM 1.075 Zr 2.627

Table 3 shows a summary of how much of each element in line numbers 1 to20 of Table 2 is jointly recovered when Pu is attempted to be recoveredat a rate of 6.93 kg/month. In Table 3, MA refers to minor actinides, RErefers to rare earths, AM refers to alkaline metals, AEM refers toalkaline earths, and NM refers to noble metals. In Table 2 the MA areAm, Cm, and Np; RE is Tb, Dy, and Sm; NM are each of the elements fromline numbers 1 to 11. AEM and NM are the elements in line numbers 21 andbelow of Table 21 and they remain in the fused fluoride without beingdeposited onto the second electrode.

Since Li₂O and BeO are added to the fused fluoride for the purpose ofconducting the reaction of expression (14), the amount of fused fluoridein the electrolysis cell increases. Accordingly, after the electrolysisusing fluoride, the increased portion of fused fluoride (excess fusedfluoride) is removed from the electrolysis cell together with the FP andthen transferred to a treatment process 6 of waste salt.

TABLE 4 Mole Component percentage of Category material element Basematerials of acidic glass SiO₂ 39.49% B₂O₃ 20.77% Al₂O₃ 4.99% Basematerials of basic glass Li₂O 10.22% CaO 2.72% ZnO 1.88% Na₂O 16.43% FPRb₂O 0.06% Cs₂O 0.26% SrO 0.15% BaO 0.16% ZrO₂ 0.60% MoO₃ 0.51% MnO₂0.22% RuO₂ 0.28% Rh₂O₃ 0.06% PdO 0.15% Ag₂O 0.01% CdO 0.01% SnO₂ 0.01%SeO₂ 0.01% TeO₂ 0.06% Y₂O₃ 0.08% Sm₂O₃ 0.08% Eu₂O₃ 0.01% Gd₂O₃ 0.01%LaO₃ 0.13% CeO₂ 0.10% Pr₆O₁₁ 0.13% Nd₂O₃ 0.42% Total 100.00% Total basematerials of acidic glass 65.25% Total base materials of basic glass31.25% Total FP 3.50%

Table 4 shows the composition, converted into mole percentages, of P0798simulated glass which is a model glass of high level waste. Basematerials of glass include base materials of acid oxide which buildnetwork structures, such as SiO₂, and base materials of basic oxidewhich sever networks and lower melting points. The base materials ofbasic oxide includes boron, which decomposes fluorides.

The excess fused fluorides (LiF and BeF₂) including FP, Li₂O, and BeO,removed from the electrolysis cell, are supplied to a high levelradiation waste treatment apparatus. In the high level waste treatmentapparatus, The FP and excess fluoride compounds are poured into asolidification vessel, the base materials of acid oxide and basematerials of basic oxide are additionally poured into the solidificationvessel, and the mixture is further agitated. Later, the solidificationvessel is sealed and the solidification vessel is stored in a sealedstate in a storage room.

TABLE 5 Converted fluoride Electrode- Component Amount of positionAmount Amount of Line Atomic Mass Radioactivity substance potentialElectric of O²⁻ increase number Element weight Form (kg) (Ci) (kmol) (V)Remarks charge consumed in salt 1 Te 127.6 TeF₆ 4.85E−02 1.34E+033.80E−04 −0.1 6 1.14E−03 2 Ru 101.07 RuF₆ 1.90E−01 4.99E+04 1.88E−03−0.142 6 5.64E−03 3 Pd 160.4 PdF₃ 8.49E−01 0.00E+00 7.98E−03 −0.214 31.20E−02 4 Rh 102.9055 RhF₃ 3.19E−01 4.99E+05 3.10E−03 −0.231 3 4.65E−035 Ag 107.868 AgF 4.21E−02 2.75E+03 3.90E−04 −0.637 1 1.95E−04 6 Mo 95.94MoF₆ 3.09E−01 0.00E+00 3.22E−03 −0.638 6 9.66E−03 7 Sb 121.75 SbF₅1.36E−03 7.96E+02 1.12E−05 −0.67 5 2.79E−05 8 Sn 121.75 SnF₄ 3.28E−023.85E+04 2.69E−04 −1.082 4 5.39E−04 9 In 114.82 InF₃ 1.09E−03 3.57E−019.49E−06 −1.104 3 1.42E−05 10 Nb 92.9064 NbF₅ 1.16E−03 5.21E+04 1.25E−05−1.19 5 3.12E−05 11 Cd 112.4 CdF₂ 4.75E−02 5.95E+01 4.23E−04 −1.316 24.23E−04 12 Tb 158.9254 TbF₃ 1.25E−03 3.02E+02 7.87E−06 −1.465Approximate 3 1.18E−05 with Yb 13 Dy 162.5 DyF₃ 6.28E−04 0.00E+003.86E−06 −1.465 Approximate 3 5.80E−06 with Yb 14 Cm 247 CmF₃ 4.70E−021.89E+04 1.90E−04 −1.505 3 2.85E−04 15 Am 243 AmF₃ 1.40E−01 1.88E+025.76E−04 −1.623 3 8.64E−04 16 Pu 244 PuF₄ 9.03E+00 1.08E+05 3.70E−02−1.733 4 7.40E−02 17 Sm 150.4 SmF₃ 6.96E−01 1.25E+03 4.63E−03 −1.819 36.94E−03 18 Zr 91.22 ZrF₄ 3.42E+00 2.77E+05 3.75E−02 −1.86 4 7.50E−02 19Np 237.0482 NpO₂F₂ 7.49E−01 1.81E+01 3.16E−03 −2.068 6 3.16E−03 20 U238.029 UO₂F₂ 1.91E+01 8.10E−02 8.02E−03 −2.253 6 8.02E−02 21 Sr 87.62SrF₂ 8.68E−01 1.74E+05 9.91E−03 −2.58 Approximate 2 0 9.91E−03 with Mg22 Ba 137.34 BaF₂ 1.20E+00 1.00E+05 8.74E−03 −2.58 Approximate 2 08.74E−03 with Mg 23 Gd 157.25 GdF₃ 6.29E−02 2.32E+01 4.00E−04 −2.823 3 04.00E−04 24 Nd 144.24 NdF₃ 3.51E+00 9.47E+01 2.43E−02 −2.584 3 02.43E−02 25 Y 88.9059 YF₃ 4.53E−01 2.38E+05 5.10E−03 −2.866 3 0 5.10E−0326 La 138.9055 LaF₃ 1.14E+00 4.92E+02 8.21E−03 −2.883 3 0 8.21E−03 27 Pr140.9077 PrF₃ 1.09E−01 7.71E+05 7.74E−04 −2.883 Approximate 3 0 7.74E−04with La 28 Pm 145 PmF₃ 1.00E−01 1.00E+05 6.90E−04 −2.883 Approximate 3 06.90E−04 with La 29 Eu 151.96 EuF₃ 1.26E−01 1.35E+04 8.29E−04 −2.883Approximate 3 0 8.29E−04 with La 30 Ce 140.12 CeF₃ 2.47E+00 8.27E+051.76E−02 −2.94 3 0 1.76E−02 31 Rb 85.4678 RbF 3.23E−01 1.90E+02 3.78E−03−3.14 Approximate 1 0 3.78E−03 with Na 32 Cs 132.9054 CsF 2.40E+003.21E+05 1.81E−02 −3.14 Approximate 1 0 1.18E−02 with Na 33 Se 78.96SeF₄ 4.87E−02 3.96E−01 6.17E−04 4 1.23E−03 34 Tc 97 TcF₆ 7.52E−021.43E+00 7.75E−04 6 2.33E−03 35 Total amount 4.79E+01 3.60E+06 2.81E−012.78E−01 9.84E−02 36 MA + FP 1.98E+01 3.49E+06 1.64E−01 37 FP 1.88E+013.47E+06 1.60E−01 38 FP mixed in 2.44E+01 1.14E+00 6.12E−02

An example of a mass balance of FP, Li, Be, etc. in the presentembodiment is explained below. Table 5 summarizes the number ofelectrons relating to the reactions of the converted fluorides shown inTable 2, the amount of O²⁻ consumed on the first electrode (anode)during the electrolysis using fused fluoride, and, for each treatment ofabout 1 t of the spent oxide form nuclear fuel, amount of substanceaccumulating in the fused fluoride for each treatment of about 1 t ofthe spent oxide form nuclear fuel.

A summary of the mass balance of FP, Li, and Be is shown in FIG. 2. Fromabout 1 t of the spent oxide form nuclear fuel generated by a lightwater reactor, 1.6×10⁻¹ kmol of the FP is introduced to the fusedfluoride in the process 3 of electrolysis using the fused fluorideprocess 3, and among this FP, 6.12×10⁻² kmol is transferred to anotherplant where is conducted the purification process 4 of embodiment 2 (orembodiment 3) described below, together with the metallic nuclear fuelmaterials (metallic uranium, metallic plutonium, etc.) recovered by theprocess of the electrolysis using fused fluoride. The remaining9.84×10⁻² kmol of the FP is discharged from the process 3 ofelectrolysis using fused fluoride to the treatment process 6 of wastesalt.

In the process 3 of electrolysis using fused fluoride, 2.78×10⁻¹ kmol ofO²⁻ is consumed, due to the recovery of the metallic nuclear fuelmaterials (metallic uranium, metallic plutonium, etc.). To supplementthese O²⁻ ions, each of Li₂O and BeO are added to the fused fluorideonly 39×10⁻¹ kmol respectively. As a result of the electrolysis usingfused fluoride, these oxides are respectively changed into LiF and BeF₂,which are the main components of the fused fluoride. The amount of thefused fluoride increases to a total of 4.17×10⁻¹ kmol, with 2.78×10⁻¹kmol of LiF and 1.39×10⁻¹ kmol of BeF₂. This increased portion of thefused fluoride (4.17×10⁻¹ kmol) is transferred to the treatment process6 of waste salt.

In order to vitrify radioactive waste generating due to the treatmentprocess 6 of waste salt, according to Table 4, with respect to 3.5% FP,65% base materials of acidic glass and 30% base materials of basic glassare required. With respect to 9.84×10⁻² kmol of the FP, 1.83 kmol of thebase materials of acidic glass and 8.79×10⁻¹ kmol of the base materialsof basic glass are required. After the process of the electrolysis usingfused fluoride completed, since Li and Be discharged from theelectrolysis cell are employed as the base materials of basic glass,4.62×10⁻¹ kmol of the base materials of basic glass must be supplied tothe electrolysis cell in the process 3 on the electrolysis using fusedfluoride.

When converted substances by fluorination generated in the fluorinationtreatment process 1 are converted into a metallic nuclear fuel materialsby using new fused fluoride without FP, since concentration of the FP inthe fused fluoride is low, the amount of the FP transferred to thetreatment process 6 of waste salt is lower than 9.84×10⁻² kmol.Nevertheless, when the concentration of the FP is increased by repeatingthe process 3 of electrolysis using fused fluoride, 9.84×10⁻² kmol ofthe FP is steadily discharged from the electrolysis cell and transferredto the treatment process 6 of waste salt. A relationship between thenumber of treatments of the process 3 of electrolysis using fusedfluoride and the amount of the FP transferred to the treatment process 6of waste salt is shown in FIG. 3. The fused fluorides contain 2 kmol ofLiF and 1 kmol of BeF₂, as initial components. Therefore, in the presentembodiment, F⁻ becomes 4 kmol. In an electrolysis method using fusedchloride, the concentrations of uranium and plutonium are about 1 to 5%.When 1 t of the spent oxide form nuclear fuel is processed in theprocess 3 of electrolysis using fused fluoride, amount of substances offluorinated uranium and fluorinated plutonium dissolved in the fusedfluoride contained in the converted substances by fluorination generatedfrom the oxide form nuclear fuel, is total 1.17×10⁻¹ kmol, from asection “amount of substance (kmol)” of the converted fluorides of Table1; thus, the necessary amount of the fused fluoride is about 0.2 to 10kmol. Accordingly, the aforementioned 4 kmol is the appropriate amountfor the present embodiment.

According to the present embodiment, since the converted fluorides andoxides generated by removing a large portion of the uranium in thefluorination treatment process tare are supplied into the fused fluoridein the electrolysis cell and the electrolysis using fused fluoride iscarried out, prior to this electrolysis using fused fluoride, there isno need for treatment to convert into the converted fluorides such asthe fluorinated uranium, fluorinated plutonium, etc. which weregenerated in the fluorination treatment process 1. Therefore, theprocess of the method for recovering metallic nuclear fuel material fromspent nuclear fuel can be simplified.

In the present embodiment, since the electrolysis using fused fluorideis conducted, the spent oxide form nuclear fuel generated in light waterreactors can be dissolved into the fused fluoride and the metallicuranium and metallic plutonium contained in this oxide form nuclear fuelcan be readily recovered.

Since the present embodiment has the fluorination treatment process 1 asthe pretreatment of the process of the electrolysis using fusedfluoride, a large portion of the uranium contained in the spent oxideform nuclear fuel can be preliminarily removed. Therefore, the apparatusfor the electrolysis using fused fluoride, which is used in the process3 of electrolysis using fused fluoride, can be made compact and theamount of the fused fluoride used can be reduced.

In the present embodiment, since oxides are added into the fusedfluoride, generation of fluorine gas near the first electrode that is ananode can be prevented and carbon gas near the first electrode can begenerated. Therefore, corrosion of the first electrode can be preventedand the life of the first electrode can be extended. In particular, inthe present embodiment, since oxides having cations common to the fusedfluoride are used, for example, as shown in expression (19), whenreacted with excess fluorine in the fused fluoride, the generatedfluorides will have the same composition as the fused fluoride.Therefore, the composition of the fused fluoride can be maintained. Whenoxides not having cations common to the fused fluoride are used, thecompositions of the fused fluoride change.

Furthermore, in the present embodiment, since a carbon electrode is usedas the first electrode that is the anode, carbon dioxide (or carbonmonoxide) is generated from the anode first electrode that is the anode.Therefore, emission of fluorine gas from the first electrode can beprevented.

Embodiment 2

A method for reprocessing spent nuclear fuel, to which the method forrecovering metallic nuclear fuel materials from spent nuclear materialsof the embodiment 1 is applied, according to embodiment 2 which isanother embodiment of the present invention, is explained by referringto FIG. 4. The method for reprocessing spent nuclear fuel of the presentembodiment is a method to which a purification process 4 and a metallicnuclear fuel manufacturing process 5 have been added to all processes ofthe method for reprocessing metallic nuclear fuel from the spent nuclearfuel of the embodiment 1, that is, added to the fluorination treatmentprocess 1, the UF₆ treatment process 2, the process of the electrolysisusing fused fluoride, and the treatment process 6 of waste salt. Sincethe fluorination treatment process 1, the UF₆ treatment process 2, theprocess 3 of electrolysis using fused fluoride, and the treatmentprocess 6 of waste salt in the present embodiment are conducted usingtreatments which are the same as these processes of the embodiment 1,explanations of these processes are omitted. The purification process 4and metallic nuclear fuel manufacturing method 5 are explained.

In the purification process 4, the recovered metallic nuclear fuelmaterials are purified by a dry reprocessing treatment applying theelectrolysis using fused chloride disclosed in, for example, MasashiKoyama et al., “Dry Reprocessing Technology”, Denchuken Review, No. 37,pp. 26-37 (2000). By conducting the dry reprocessing treatment applyingthis electrolysis using fused chloride, a metallic nuclear material witha composition in which all high level radiation NM were removed from themetallic nuclear fuel materials with the composition shown in Table 3can be obtained. The obtained metallic nuclear fuel has the compositionshown in Table 6.

TABLE 6 Amount required for manufacturing fresh Type of metallic fuelassemblies nuclear fuel (kg/month) Component U 26.44 Pu 6.92 MA 0.18 RE0.089 AM 0 AEM 0 NM 0 Zr 3.737

Table 6 shows a metallic nuclear fuel used in the manufacture of freshfuel assemblies. This metallic nuclear fuel shown in Table 6 is shown inFIGS. 4-5-1 on page 37 of Masashi Koyama et al., “Dry ReprocessingTechnology”, Denchuken Review, No. 37, pp. 26-37 (2000).

TABLE 7 Type of metallic Fresh fuel nuclear fuel assembly Component U3.82081 Pu 1 MA 0.02601 RE 0.01286 AM 0 AEM 0 NM 0 Zr 0.54003

Table 7 has the metallic nuclear fuel components of Table 6, butrewritten based on a Pu amount of 1. The fresh fuel assembly havingmetallic nuclear fuel with the composition shown in Table 6 includes aquantity of U which is 3.82 times that of Pu. In the fluorinationprocess 1, U is removed such that the amount thereof will be 3.82 orless that of the amount of Pu contained in the converted fluoridesupplied to the process 3 of electrolysis using fused fluoride.

The metallic nuclear fuel (in Table 3, metallic nuclear fuel having acomposition with 0 for NM) obtained in the purification process 4becomes nuclear fuel substance to be loaded into each fuel rod includedin the fresh fuel assemblies to be used in a fast breeder reactor. Whenthe composition of the metallic nuclear fuel obtained in thepurification process 4 is compared with the composition (Table 6) of themetallic nuclear fuel required for the fresh fuel assemblies, themetallic nuclear fuel obtained in the purification process 4 becomesless than that of U and Zr shown in Table 6. In the metallic nuclearfuel obtained in the purification process 4, Pu, MA, and RE fulfill thespecifications shown in Table 6.

In the metallic nuclear fuel manufacturing process 5, the fuel rods aremanufactured by using a metallic nuclear fuel manufacturing apparatus.That is, using the metallic nuclear fuel obtained in the purificationprocess 4, as well as metallic uranium and metallic zirconium added suchthat the amount of each of U and Zr will be that shown in Table 6, aplurality of fuel pellets are manufactured. These fuel pellets areloaded into a fuel cladding and then the fuel rods are manufactured bysealing the fuel cladding. As the metallic uranium to be added, forexample, one part of the UF₆ generated in the fluorination treatmentprocess 1 is used, as metallic uranium. Metallic uranium manufacturedbased on depleted uranium generated by uranium enrichment may be used asthe metallic uranium to be added.

A plurality of fuel rods bundled together by a plurality of fuel spacersare attached to a lower tie plate, the fuel rod bundle is passed througha wrapper tube, and a lower end portion of the wrapper tube is attachedto the lower tie plate. In this way, the fresh fuel assemblies to beloaded into the core of the fast breeder reactor are manufactured.

The present embodiment can also achieve each effect obtained in theembodiment 1. According to the present embodiment to which the methodfor recovering metallic nuclear fuel materials from the spent nuclearfuel materials of the embodiment 1 is applied, the process for themethod for reprocessing spent nuclear fuel can be also simplified.

Embodiment 3

A method for reprocessing spent nuclear fuel, to which the method forrecovering metallic nuclear fuel materials from spent nuclear materialsof the embodiment 1 is applied, according to embodiment 3 which isanother embodiment of the present invention, is explained by referringto FIGS. 5 and 6. In the method for reprocessing spent nuclear fuel ofthe present embodiment, each process shown in FIG. 4 is carried out. Theprocess 3 of electrolysis using fused fluoride of the present embodimentis explained in detail based on FIG. 5 and the purification process 4 isexplained in detail based on FIG. 6.

In the process 3 of electrolysis using fused fluoride of the presentembodiment, a fused fluoride electrolysis apparatus 10, shown in FIG. 5,is used. The fused fluoride electrolysis apparatus 10 is provided withan electrolysis cell 11, a first electrode (electrode for electrolysis)13, which is an anode, and a second electrode (first recovery electrode)14, which is a cathode. For example, a circular dam 12 is installed ona, bottom surface of the electrolysis cell 11. The second electrode 14is disposed at the inner side of the dam 12 in the electrolysis cell 11.The first electrode 13 facing the second electrode 14, is arranged abovethe dam 12 in the electrolysis cell 11. The first electrode 13 and thesecond electrode 14 do not contact the dam 12. As the fist electrode 13,for example, a carbon electrode is used and for the second electrode 14,for example, an iron electrode is used. Fused fluoride 15, which is amixture of LiF and BeF₂, is filled into the electrolysis cell 11, and aliquid surface of the fused fluoride 15 reaches above the upper end ofthe dam 12. The first electrode 13 and the second electrode 14 areimmersed in the fused fluoride 15. Converted fluoride 17 obtained in thefluorination treatment process 1 is poured into an area outside of thedam 12 in the electrolysis cell 11. The converted fluoride 17 isdissolved into the fused fluoride 15.

A mixture of Li₂O and BeO that are oxide is added to the fused fluoride15. Similar to Embodiment 1, a voltage is applied between the firstelectrode 13 and the second electrode 14 such that the potential of thesecond electrode 14 provided in the electrolysis cell will be equal tothe electrodeposition potential of the element (uranium), among theelements required for the metallic nuclear fuels being manufactured, forwhich the electrodeposition potential of the fused fluoride is lowest.The elements having at least the electrodeposition potential of theelement with the lowest electrodeposition potential will deposit ontothe second electrode 14. The composition of the metallic nuclear fuelmaterial deposited on the second electrode 14 is as shown in Table 3.Carbon oxide gas is generated from the first electrode 13.

After the process 3 of electrolysis using fused fluoride was finished,in order to remove metallic impurities (shown in Table 2, from Te inline number 1 to Cd in line number 11) deposited onto the secondelectrode 14, the purification process 4 is carried out. Prior to carryout this purification process 4, the first electrode 13, which was usedin the process 3 of electrolysis using fused fluoride, is removed and afresh first electrode (second recovery electrode) 16 is attached to theelectrolysis cell 11 (see FIG. 5). The first electrode 16 is alsoimmersed in fused fluoride 15.

In the process 3 of electrolysis using fused fluoride, the amount ofuranium in the metallic nuclear fuel material deposited on the secondelectrode 14 is the greatest. In the purification process 4, withoutapplying a voltage to the first electrode 16 and the second electrode16, the electric potential of the second electrode 14 approaches theelectrodeposition potential of uranium. Then, when a slight electricpotential difference is applied between the first electrode 16 and thesecond electrode 14 such that the first electrode 16 becomes negative,the electric potential of the second electrode 14 becomes slightly morepositive than the electrodeposition potential of uranium. Therefore, theuranium deposited on the second electrode 14 begins to dissolve into thefused fluoride 15. Since the first electrode 16 is more negative thanthe second electrode 14, the uranium dissolved in the fused fluoride 15begins to deposit onto the first electrode 16. When the electricpotential difference applied to the first electrode 16 and the secondelectrode 14 increases, the electric potential of the second electrode14 then progressively becomes positive and rises above theelectrodeposition potential of Np. At this time, the Np deposited on thesecond electrode 14 dissolves into the fused fluoride 15 and thedissolved Np deposits onto the first electrode 16. In this way, byadjusting the electric potential difference applied between the firstelectrode 16 and the second electrode 14 such that the value of theelectric potential of the second electrode 14 will become more positivethan that of the electrodeposition potential of Tb, and theelectrodeposition potential will become a more negative value than thatof the electrodeposition potential of Cd, each element deposited ontothe second electrode 14 and having an electrodeposition potential ofthat from Tb to U dissolves into the fused fluoride 15 and then depositsonto the first electrode 16. At this time, each element having anelectrodeposition potential of that from Te to Cd deposits onto thesecond electrode and then remain. In the purification process 4 of thepresent embodiment, the first electrode 16 becomes the cathode and thesecond electrode 14 becomes the anode. Due to the abovementioned, thepurification process 4 finishes, and the metallic nuclear fuel havingthe same composition as when the purification process 4 of theembodiment 1 finishes can be obtained. In the purification process 4,each element remaining on the second electrode 14 may be recovered, thesecond electrode 14 being takenout from the electrolysis cell 11 andcleansed, as required.

After the purification process 4 finished, the first electrode 16 istaken out from the electrolysis cell 11, and this first electrode 16 istransported to the metallic nuclear fuel manufacturing apparatus of themetallic nuclear fuel manufacturing process 5. Using the metallicnuclear fuel deposited onto the first electrode 16, similar to theembodiment 1, the fresh fuel assembly is manufactured by the metallicnuclear fuel manufacturing apparatus.

The present embodiment can obtain each of the effects which generate inthe embodiment 2. The present embodiment can conduct the purificationprocess 4 by using the fused fluoride electrolysis apparatus 10,exchanging the first electrode 16 for the first electrode 13 with thefused fluoride 15 loaded as it is in the electrolysis cell 11 of thefused fluoride electrolysis apparatus 10 used in the process 3 of fusedfluoride electrolysis, and making the first electrode 16 the cathode andthe second electrode 14 the anode and applying an electric potential.That is, since the present embodiment can carry out the process 3 ofelectrolysis using fused fluoride and the purification process 4 byusing the fused fluoride electrolysis apparatus 10, the process of themethod for reprocessing spent nuclear fuel can be made even more simplethan that of the embodiment 1 which conducts the process 3 ofelectrolysis using fused fluoride with using a fused fluorideelectrolysis apparatus, and the purification process 4 with using afused chloride electrolysis apparatus. The present embodiment does notrequire the use of a fused fluoride electrolysis apparatus differentfrom one like that in the embodiment 1.

The fused fluoride electrolysis apparatus 10 used in the process 3 ofelectrolysis using fused fluoride and purification process 4 is oneexample, and as this apparatus, an apparatus, which applies anodeelectrolysis to the metal deposited onto the second cathode 14 by theelectrolysis using fused fluoride, and deposits dissolved metal onto adifferent electrode, may be used. For example, in the fused fluorideelectrolysis apparatus 10, although the first electrode 13 was exchangedwith the first electrode 16, a fused fluoride electrolysis apparatusprovided with an electrolysis cell 11 with the first electrodes 13 and16 and second electrode 14 may be used. In this case, in the process 3of electrolysis using fused fluoride, as previously stated, the electricpotential difference between the first electrode 13 and the secondelectrode 14 may be adjusted, and in the purification process 4, asstated above, the electric potential difference between the firstelectrode 16 and the second electrode 14 may be adjusted.

The principles of the embodiment 3, when consolidated, are as follows.The fused fluoride in which the spent oxide form nuclear fuel isdissolved includes a plurality of recovered elements (called “group Aelements”), a plurality of elements which more readily dissolve into thefused fluoride than each element of group A (called “group B elements”),and a plurality of elements which less readily dissolve into the fusedfluoride than each element of group A (called “group C elements”). Whenthe first electrode is made an anode and the second electrode is made acathode and a voltage is applied between the electrodes, and theelements of the group A deposit onto the cathode second electrode fromthe fused fluoride, the elements of the group C also deposit onto thesecond electrode and the elements of the group B do not deposit onto thesecond electrode. When a voltage is applied between the electrodes suchthat the second electrode becomes an anode and the first electrodebecomes a cathode, each element of the group A adhering to the secondelectrode deposits onto another first electrode after dissolving intothe fused fluoride. Nevertheless, each element of the group C remainadhered to the second electrode, not dissolving into the fused fluoride.That is, each element of the element group for which recovery is desiredand contained in the spent oxide form nuclear fuel can be separated in ametallic state by electrolyzing metals for which recovery is desired andby transferring them another electrode among the elements that oncedeposit onto a certain electrode.

1. A method for recovering metallic nuclear fuel materials from spentnuclear fuel, comprising steps of: generating nuclear fuel fluorides byreacting fluorine with spent oxide form nuclear fuel taken out from anuclear reactor, removing one part of fluorinated uranium from amongsaid nuclear fuel fluorides, dissolving the remaining nuclear fuelfluorides and oxides into fused fluoride, and energizing a firstelectrode which is an anode and a second electrode which is a cathode,both of which were immersed into the fused fluoride, and depositing ontothe second electrode a metallic nuclear fuel material dissolved in saidfused fluoride.
 2. The method for recovering metallic nuclear fuelmaterials from spent nuclear fuel according to claim 1, wherein theoxide is an oxide having a cation contained in the fused fluoride. 3.The method for recovering metallic nuclear fuel materials from spentnuclear fuel according to claim 1, wherein an electrode containingcarbon is used as the first electrode.
 4. A method for reprocessingspent nuclear fuel, comprising steps of: generating nuclear fuelfluorides by reacting fluorine with used oxide form nuclear fuel takenout from a nuclear reactor, removing one part of fluorinated uraniumfrom among said nuclear fuel fluorides, dissolving the remaining nuclearfuel fluorides and oxide into fused fluoride, and energizing a firstelectrode which is an anode and a second electrode which is a cathode,both of which have been immersed into the fused fluoride, and depositinga metallic nuclear fuel material dissolved in the fused fluoride ontothe second electrode, and purifying the metallic nuclear fuel materialdeposited onto the second electrode.
 5. The method for reprocessingspent nuclear fuel according to claim 4, wherein the oxide is an oxidehaving a cation contained in the fused fluoride.
 6. The method forreprocessing spent nuclear fuel according to claim 4, wherein thepurification of the metallic nuclear fuel material is carried out byusing a fused chloride electrolysis apparatus having an electrolysiscell filled with a fused chloride.
 7. The method for reprocessing spentnuclear fuel according to claim 4, wherein the purification of themetallic nuclear fuel material is carried out by, energizing the secondelectrode and a third electrode such that the second electrode becomes acathode and the third electrode becomes an anode when the metallicnuclear fuel material deposited onto the second electrode is immersedinto the fused fluoride, and the third electrode is immersed into thefused fluoride; and depositing a plurality of materials used in themanufacture of metallic nuclear fuel, among materials contained in saidmetallic nuclear fuel materials, onto the third electrode.
 8. The methodfor reprocessing spent nuclear fuel according to claim 4, Wherein whenthe metallic nuclear fuel material deposited onto the second electrodeincludes a first metallic material used in the manufacture of themetallic nuclear fuel and a second metallic material which less readilydissolves into the fused fluoride than the first metallic material andwhich is unnecessary for the manufacture of the metallic nuclear fuel,and the fused fluoride includes a third metallic material which moreeasily dissolves into the fused fluoride than the first metallicmaterial, a third electrode is immersed into the fused fluoride with themetallic nuclear fuel material deposited onto the second electrode beingimmersed in the fused fluoride; and the purification of the metallicnuclear, fuel material is carried out by generating between the secondelectrode and the third electrode an electric potential in which thefirst metallic material deposited onto the second electrode is dissolvedinto said fused fluoride, in which the second metallic nuclear materialdeposited onto the second electrode is not dissolved into said fusedfluoride, in which the first metallic material dissolved in the fusedfluoride deposits onto the third electrode, and in which the thirdmetallic material does not deposit onto the third electrode the thirdmetallic material.
 9. The method for reprocessing spent nuclear fuelaccording to claims 4, wherein an electrode containing carbon is used asthe first electrode.